Yep sounds seized check the piston as said before, if the bike is locked up and wont turn the engine whilst pushing and engaged in gear then its never good. I have sent the kickstart mech break and jam in side the casing but check piston first. Is it the 2 stroke oil in the tank behind the right rad scoop model??
If it is the worst case and the engines looking bad, got two dt125 engines avaialble pm me for deatils. Might be able to get you back on the thing cheap with a swap.